Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister containing a suitable adsorbent, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to an engine intake for combustion, further improving fuel economy.
In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel canister. On a boosted engine, that vacuum draw may be supplied via an ejector during boosted operation. Simultaneously, a canister vent valve coupled between the fuel canister and atmosphere is opened, allowing for fresh air to enter the canister. Further, in some examples a vapor blocking valve coupled between the fuel tank and the fuel canister is closed to prevent the flow of fuel vapors from the fuel tank to the engine. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, reduced engine operation times in hybrid vehicles can lead to insufficient purging of fuel vapors from the vehicle's emission control system. For example, regions of adsorbent that see relatively less air flow may retain relatively more hydrocarbons. The residual hydrocarbons may desorb over a diurnal cycle, leading to an increase in bleed emissions. Additionally, the capability of the canister to trap additional vapors from the fuel tank greatly depends upon how thoroughly the vapors are purged from the canister when the vehicle was last operated. Accordingly, it is desirable to purge the canister as much as possible while the engine is running.
As such, due to the shorter purge times available in hybrid vehicles, purge operations tend to be more aggressive with higher purge ramp rates (relative to corresponding non-hybrid vehicles). In one example, U.S. Pat. No. 6,202,632 B1 teaches the use of a controllable canister purge valve comprising a first connection to the intake pipe of an engine and a second connection to a vapor canister, the first and second connections interconnected via a first controllable valve, as well as through a second valve connected in parallel with the first. The second valve may comprise a larger cross section than the first valve, such that greater flow rates may be achieved via the opening of both the first and second valves, in one example.
However, the inventors herein have recognized an issue with the above approach. High purge rates can cause a significant pressure drop across the canister, thus putting the fuel tank at a vacuum. For example, during a purging operation a vacuum blocking valve (VBV) may be closed to prevent flow of fuel vapors from the fuel tank to the engine. In some examples the VBV may have a small vapor path around it, intended to let fuel vapor escape slowly as the tank pressurizes with heat gain and thus avoid pressure build. Thus, with high purge rates, vacuum may develop in the fuel tank via the vapor line around the VBV, even with the VBV closed. If the purge rate is high enough, fuel tank vacuum may become high enough to overcome the closing force of the fuel tank relief valve, thus air and dirt particles may be drawn into the tank, and this flow additionally does not serve to purge the canister.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, a method is provided, comprising during a first condition, including an engine-on condition, closing a VBV and directing vapor flow from a fuel tank to a fresh air side of a vapor canister via a first vapor line, and during a second condition, including a refueling event, opening the VBV and directing vapor flow from the fuel tank to a load side of the vapor canister via a second vapor line. In this way, VBV functionality is preserved while vacuum generated at the fuel tank may be limited during conditions of high canister purge flow rates. As one example, during a purging operation a VBV may be maintained in a closed conformation. Because the fuel tank is coupled to the vapor canister on the fresh air side, not on the load/purge side, the vacuum imposed on the fuel tank may be shallow as compared to a case in which the fuel tank is coupled to the vapor canister on the load/purge side. As such, high purge flow rates may be applied to purge the canister while low fuel tank vacuum is maintained. In this way, vapor canister purging may be made more efficient, thus reducing evaporative emissions. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.